The Geology and Building Stones of
Burton Bradstock: Jo Thomas, September, 2000.

The building stones used in Burton Bradstock come almost entirely from
within the parish. All the stones used are Jurassic (about 180 million
years old) and were formed as sediments of clay, sand or lime mud under a
shallow sea on a continental shelf.

The cliffs at Burton Bradstock show three contrasting profiles. On the
west the golden yellow Bridport Sands were formed as a huge sandbank. Hard
bands were created occasionally when the top of the sandbank was in such
shallow water that the lime precipitating from seawater in the subtropical
climate cemented the grains together. As conditions changed sea level
remained shallow for a long time, possibly up to 8 million years, and
several different beds of limestone formed. These are known as Inferior
Oolite Limestone, and can be seen at the top of the cliff. The lower beds
are too sandy and the middle ones too full of fossils to use as building
stone, but the top limestones were quarried on the slope of the hill at
Southover. A small patch of the lowest Fullers Earth clay tops the
cliff near Freshwater.

West Cliff

Sequence of rocks on the
coast near Burton Bradstock

East Cliff

The eastern cliff shows the upper part of the Fullers Earth Clay.
The whole sequence of lime-rich clay formed in deeper water. As more
continental movement caused sea level to fluctuate, the character of the
clay changed and occasional banks of broken shells formed in shallower
water. These shell-banks have become the Forest Marble limestone that has
been quarried on North Hill and on Cogden Farm.

The sides of Cliff Hill show a cut through the Inferior Oolite and are a
Site of Special Scientific Interest.

On Cliff Hill the Top Limestones
are hidden by vegetation in the summer.

The Jurassic rocks of the Bridport area were subject to enormous pressure during
the movement of continents about 140 million years ago. Large sections broke
away and moved sideways or vertically along faults. As the blocks moved, the
edges broke into small pieces that were dragged along in the fault zone. When
this phase of movement ceased, the broken pieces were cemented together by later
sediments. The central part of Burton beach is a fault zone, and forms a weak
point in the shoreline. The fault can be traced north eastwards because Larkfield
was a quarry in the Inferior Oolite on the west of the fault, and along the
Bredy road is a quarry in the Forest Marble, on the east of the fault.

The fault and the structure of rocks inland are illustrated below:

Difference across the fault line

Sketch map of outcrops of Middle Jurassic
at Burton Bradstock (with apologies to British Geological Survey).

Looking north from the west cliff top, the village is built on Fullers
Earth Clay over the Inferior Oolite, the lower slopes of North Hill are the
remainder of the Fullers Earth Clay, and the top is capped by the Forest
Marble limestone. Old
maps show some of the quarries where the stone was extracted from North
Hill and the long redundant workings can still be seen today.

Large Forest Marble
cobbles can be found on the eastern beach.

The Bride river has cut a valley into the soft clay.

Both the Inferior Oolite and the Forest Marble have been used in the village
buildings. As Forest Marble is a stronger limestone, in which calcium carbonate
(lime) from dissolved shells has recrystallised to cement together the remaining
broken shells, it is used as a foundation and damp-proof course even when the
remaining walls are of the Inferior Oolite. Examples of building stone used
in the village are shown below:

Cottage built of a mix of Inferior Oolite
and Forest Marble

The White House is built of Forest Marble,
with Inferior Oolite quoins.

Cottage almost entirely of Inferior Oolite

An older house of Forest Marble has an 18th
century brick front. The bricks may have been made from Oxford Clay in Bothenhampton,
or from Down Cliff Clay at Allington

.Mill Cottages have Forest Marble in the lower
part of the wall and Inferior Oolite above that must have come from the
excavated land on which they are built

About Jo Thomas

Mrs. JO Thomas of Broadstone kindly put together this article on the geology
and building stones of the village. JO is Secretary to Dorset's Important Geological
Sites group, which includes the Dorset Geologists' Association group, Dorset
Environmental Records Centre (DERC) http://www.derc.org.uk,
the Dorset Wildlife Trust http://www.wildlifetrust.org.uk/dorset,
the Dorset County Museum, English Nature, and representatives of the District
and County Councils http://www.dorset-cc.gov.uk.
She claims to be an enthusiastic amateur but her knowledge seems to have no
limits!Author: JO Thomas - September, 2000